ATM telecommunications systems and method for routing narrow band traffic

ABSTRACT

A telecommunications system comprises an asynchronous transfer mode (ATM) network having uncommitted bandwidth, and a plurality of adaptive grooming routers (AGR) coupled to the network. The AGRs comprise a group adapted to function as a virtual transit exchange whose fabric and control are distributed over the group. The vitual comprising the AGRs incorporates independent connection control and call routing functions and has means for determining the current system status whereby to set up narrow band connections across the ATM network based on that status determination.

TELECOMMUNICATIONS SYSTEM

This invention relates to digital communications systems and inparticular to systems embodying asynchronous transfer mode (ATM)technology.

BACKGROUND OF THE INVENTION

The asynchronous transfer mode (ATM) technology is a flexible form oftransmission which allows any type of service traffic, voice, video ordata, to be multiplexed together on to a common means of transmission.In order for this to be realised, the service traffic must first beadapted typically into 53 byte cells comprising 5 byte headers and 48byte payloads such that the original traffic can be reconstituted at thefar end of an ATM network. This form of adaptation is performed in theATM adaptation layer (AAL).

A discussion of ATM adaptation of narrow band traffic is given inspecification No GB-A-2,290,433 (EP 9411944) which describes a systemand method in which an adaptive virtual junctor is used to adapt an ATMswitch to perform a narrow band .e.g. 64 kb/s, switching functionwhereby to carry narrow band services on a broadband network

As telecommunications networks increase in complexity and carryincreasing volumes of traffic, the current procedures for setting upconnections between subscribers are limiting the performance of thesenetworks. In particular, congestion may be caused by attempting toconnect to a subscriber who is already busy, or by attempting to choosea route through an already congested part of the network. Thus equipmentand resources can be wasted in attempts to set up calls which cannot becompleted. A further problem is that of scalability. As the networkexpands to accommodate increased traffic and a larger number ofsubscribers, there is an increasing need to facilitate integration ofnew equipment into an existing network without simply increasing thecongestion problem.

SUMMARY OF THE INVENTION

The object of the invention is to minimise or to overcome thesedisadvantages.

According to one aspect of the present invention there is provided adistributed telecommunications exchange system having independent callrouting and connection control for setting up connections across thesystem.

According to another aspect of the invention there is provided atelecommunications system, including an asynchronous transfer mode (ATM)network, and a plurality of adaptive grooming routers (AGR) coupled tothe network, wherein the AGRs comprise a group adapted to function as asingle distributed or virtual transit exchange whereby in use to set upnarrow band connections across the ATM network.

According to one aspect of the present invention there is provided adistributed telecommunications exchange system having means fordetermining the current status of the system whereby to effect routingof narrow band traffic across the system.

According to another aspect of the invention there is provided a methodof communicating resource availability to maintain performance of adistributed exchange system under overload conditions.

According to a further aspect of the invention there is provided amethod of routing telecommunications traffic in a system including anasynchronous transfer mode (ATM) network having uncommitted bandwidth,and a plurality of adaptive grooming routers (AGR) coupled to the ATMnetwork, which AGRs comprise a group adapted to function as a virtualtransit exchange whose fabric and control are distributed over thegroup, the method including determining the current system statuswhereby to set up narrow band connections across the ATM network basedon that status determination.

The technique provides for the separation of call routing and connectioncontrol together with the advertising of the system status. This ensuresa wide range of scaleability so that the application of dynamic trunkingtechnology provides scaleability in a traffic sense. Further, theseparation of call routing and connection control provides a distributedcomputing environment which is scaleable and managed by thisadvertisement resource. Because the distributed exchange manages its owninternal traffic, effectively it provides means for balancing thattraffic to the fabric and makes its own internal routing decisions.

Reference is here directed to our co-pending United Kingdom patentapplications Nos. 9410294.4, 9410295.1, 9411944.0 and 9502552.4 whichrelate to arrangements and methods for handling narrow band traffic inan ATM communications network.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to theaccompanying drawings in which:

FIG. 1 illustrates an AGR network system that is the preferredembodiment of a large distributed exchange system;

FIG. 2 illustrates signalling separation in the network of FIG. 1:

FIG. 3 illustrates the provision of advanced services in the network ofFIG. 1;

FIG. 4 illustrates the principle of dual homing and diverse path routingexternal narrow-band exchanges to the distributed exchange system;

FIG. 5 illustrates the connection of the large distributed exchange to ahierarchy of narrow-band exchanges;

FIG. 6 illustrates an overview of handling a connection set-up requestand the roles of call routing, connection control and voice-pathconnection termed wormholes;

FIG. 7 illustrates the principle of using existing N-ISUP signalling asthe means for routing calls across the distributed exchange system atthe Call Routing layer;

FIG. 8 illustrates the principle of controlling routing and congestionin the Call Routing and Connection Control layers;

FIG. 9 shows a AGR protocol stack for the network of FIG. 1;

FIG. 10 illustrates a means for specifying a speech path across thedistributed exchange system;

FIG. 11 illustrates the functional topology of the physicallydistributed exchange system;

FIG. 12 illustrates an arrangement for determining available trunkcapacity on a Virtual Trunk Route (VTR);

FIG. 13 illustrates an arrangement for determining a Virtual TrunkGroup's (VTG) capacity;

FIG. 14 illustrates an arrangement for advertising congestion on aremote node's VTG in the distributed exchange system;

FIG. 15 illustrates an arrangement for advertising congestion on aremote node's VTR in the distributed exchange system;

FIG. 16 illustrates an extension of the arrangement of FIG. 13 forspecifying available voice band processor capacity;

FIG. 17 illustrates a preferred signalling process to create a speechpath between AVJs in the network of FIG. 1;

FIG. 18 illustrates the information content required to establish orremove a narrow-band connection;

FIG. 19 illustrates a method for tracing a narrow-band connectionthrough the large distributed exchange from a trunk circuit; and

FIG. 20 illustrates the overall protocol for establishing a connectionend to end in the layers Call Routing and Connection Control and atphysical layers;

DESCRIPTION OF PREFERRED EMBODIMENT

Referring first to FIG. 1, this depicts the functional architecture ofthe Adaptive Grooming Router (AGR) network system. The AGR works on theprinciple of a single fabric switch (SFS) incorporating a novel devicean adaptive virtual junctor (AVJ), that adapts narrow-band traffic toand from the ATM adaptation layer e.g. (AAL-1), and also provides themeans to time switch narrow-band channels. Using a standard ATMswitching fabric, the AVJ allows a fully scalable narrow-band exchangeto be built, by communicating through the switch fabric using a fullyinterconnected mesh of dynamically sized trunk groups or virtual channeljunctors (VCJ), which gives a non-blocking space switching performance.The AGR extends the scale of this principle to the wide area by usingdynamically sized Virtual Trunk Groups VTGs to construct a fullyinterconnected mesh of AGRs that can dynamically adapt to thecommunity-of-interest in the wide area network. The network of AGRs istermed an AGR network system, and functions as a large distributednarrow-band exchange, that at a minimum can replace a single transitexchange, and at a maximum replace the existing trunk/transit layers ofan hierarchical narrow-band exchange network. The term Adaptive GroomingRouter (AGR) is used since the node grooms traffic to/from localexchanges according to instantaneous demand for a trunk/transitconnection. The VCJs and VTGs function on the principle of dynamicstructured data transfer (DSDT) to provide a end-to-end narrow-bandconnection capability, and the ability to perform path tracing.

The capacity of a single AGR node is dependent on the size of the hostATM switch, the number of connected AVJ devices and the capacity of theAVJ device itself. Further, the AVJ device can be dedicated to performeither a trunking function (termination of synchronous narrow-bandcircuits) or a grooming function (generation of traffic groups fortransmission across the WAN) equivalent to a tandem switching functionof NB networks. Therefore both the overall capacity of the node and themix of traffic (intra and inter node) can be dimensioned according toneed. For example, consider a node built using 8,000 channel AVJdevices, hosted to a 10 Gb/s ATM switch. A total of 11 fullyinterconnected AVJ devices with four dedicated to trunking and 7 togrooming yields node with a total of 70,000 circuits (32,000 TDM trunksand 28,000 WAN trunks). In this configuration the node can terminate upto 28,000 Erlangs of traffic from the synchronous local exchange networkwith over 80% of this traffic capable of being carried across the WAN.If however, the amount of WAN traffic is less, then one or more of thegrooming AVJ devices could be sacrificed for a further trunking AVJ thusincreasing the amount of synchronous trunking capacity.

For AGR network resilience, a minimum of two VTGs will interconnect allof the nodes within a network. The capacity of the routes can bedimensioned according to the communication demand—high capacity VTGsconnecting high traffic routes, 47 trunks and up and low capacity VTGsconnecting low demand routes 6-46 trunks. In this manner the overallgrooming capacity of a node can be flexibly dimensioned to connect toeither a very large number of nodes (with modest traffic on each route)or to a lower number of nodes (but with a large amount of inter-nodaltraffic or a mix of both. As an example, consider the node describedabove, with 28,000 inter-nodal trunks. In one configuration this couldsupport 178 similar nodes each node interconnected by two VTGs with anaverage capacity of 80 trunks (the capacities of the individual VTGs canof course dynamically increase and decrease according to demand). Inthis manner a network capacity of over 5,000,000 narrow-band trunks canbe supported with over 80% of this traffic able to traverse the WAN atany one time.

FIG. 1 also shows the manner in which the AGR node provides e.g. N-ISDNSS7 signalling capability and call routing.

The arrangement provides a means for establishing, maintaining andremoving connections in an AGR network. The AGR network system iscontrolled from a proxy call routing server in an ATM network to providenew services to users in the network, stemming directly from theaforementioned separation of call routing from connection control whichis illustrated in FIG. 2 which illustrates the manner in whichadaptating signalling physical layers to ATM allows evolution from STPbased signalling to signalling as an ATM network service.

FIG. 3 illustrates the principle of an AGRNS that provides a connectionengine, controlling narrow-band connections between endpoints specifiedby the local Call Routing subsystem, or by a proxy server. Therefore theproxy server may implement signalling systems that go beyond thecapabilities of the N-ISDN SS7 signalling subsystem in the AGR node, andprovide for example multimedia services and control thepoint-to-multipoint hardware capability of the AGR. In the exampleillustrated, the D channels of ISDN Primary Rate Access are routed to aframe processor located in the ATM network, accessed by a VirtualChannel Connection. The frame processor takes HDLC frames and adaptsthem to/from AAL-5. The D channels may carry HDLC frames that areadapted to the broadband MTP-2 protocol Signalling ATM Adaptation Layer(SAAL) and then routed to the proxy server. The HDLC frames may containany layer 3 or higher layer of the OSI model as signalling messages fordirect interpretation by the software within the proxy server. Thesemessages are translated into commands that use the ApplicationsInterface of the connection control of the AGR node or are formed intoSS7 messages or any other suitable proxy signalling protocol that can beinterpreted by the Connection Control layer of the AGR software toinstruct it to control narrow-band connections between specifiedendpoints. By separating the service aspect from the connection aspectin this manner, new services may be made available in the network.

Referring now to FIG. 4, this illustrates the principle of dual homingof narrow-band exchanges to the AGR network system. In hierarchicalnarrow-band transit networks, a local exchange in the lowest tier mustprioritise its traffic across many small routes depending ongeographical destination of a call and traffic congestion levels. Sincethe AGR network system performs a grooming and dynamic handling oftraffic demand, the local exchange can now simply “hook on” to the AGRnetwork system using a large voice route. The large voice route hasincreased Erlang efficiency, and providing two large voice routesprovides a degree of resilience if the traffic is balanced across thetwo routes. The AGR network system can also make use of the dual homingfacility to balance traffic internally, since there is a choice ofdestination AGR when communicating with any dual homed narrow-band localexchange.

FIG. 5 illustrates the connection of the AGRNS into an existingnarrow-band network hierarchy of exchanges. In such a narrow-bandnetwork it is traditional to provision the network to have a choice ofroutes with priority given to the most direct route between exchanges.However the routes will be sized such that they experience a differingdegree of blocking probability that decreases with decreasing priority,such that a backbone route via exchanges in the highest tier has a muchlower blocking probability than routes directly between local exchanges.The AGRNS may be deployed to replace just one such exchange in tiers 2or 3, or to provide a backbone narrow-band network using ATM transportthat could replace part or all of the narrow-band transport andexchanges. This would ultimately form a single tier transit network.Consequently, the AGRNS must in general be able to cope with theprioritisation of voice routes that connect it to the narrow-bandtransit hierarchy. These voice routes would suffer congestion because ofoccupancy and also exchange processor activity, as described in the ITUWhite Book Standards on SS7. Internally the AGRNS must cope with theshared utilisation of the ATM transport and with the blocking caused byfragmenting its grooming capacity across a number of physical devices.The load balancing arrangement shown in FIG. 5 exploits the dual ormulti-homed connectivity in the narrow-band network to even out trafficoccupancy on external voice routes and internal trunk groups so as delaythe onset of congestion and maintain carried traffic levels underoverload conditions by rejecting offered traffic at the periphery of theAGRNS.

Reference is now made FIGS. 6 to 20 which illustrate stages of theconnection set up in the network of FIG. 1 in response to a set uprequest. The diagram of FIG. 6 shows two separate functions, callrouting and connection control. FIG. 6 also shows the stages ofprogression of the call. The incoming initial address message (IAM),which may comprise the initial address message of the number 7signalling system, goes to call routing which makes decisions based on adigit analysis by using as many digits as it needs to determine theoutgoing voice route or set of voice routes in the narrow-band network(see FIG. 2b) and it may, if necessary, wait for other signallingmessages. Once call routing has chosen the candidate set of voice routesit can then make decisions based on static prioritisation of those voiceroutes. I.e. a voice route through a local exchange can be used inpreference to one through a transit exchange, which preferably is usedless often. The call routing then makes an assessment of congestion onthose voice routes. This could either be a total circuit occupancy ofthe voice route or it could be congestion associated with an exchangeand the processing power it has available to handle signalling messages,e.g. in accordance with ITU standards on SS7. It will be appreciatedthat congestion can arise both from excess voice route utilisation whichis using up circuits in those voice routes and from exchange loadingwhich is a congestion indication which is exchange specific but notnecessarily associated with the voice route which is to go between theAGR network system and the exchange.

Connection control is responsible for ensuring that a connection can bemade between the end points P and Q through the AGR network system.Messages can be passed from call routing (CR) to connection control (CC)which can then eliminate routes with respect to internal congestion, ormay be continually passed from connection control to CR such that callrouting may eliminate candidate routes in priority order. Internalcongestion can be due to having to share the ATM network with othertypes of traffic so that voice traffic demands have to compete withother demands for the same resources, and also because inherently in thedesign of the AGR there is a fragmentation of resources in the physicaldevices. Whilst there may be a certain amount of capacity available on apooled resource, it may in fact be fragmented over a number of physicaldevices and therefore steps should be taken to balance the load betweenthe physical devices. It is possible to arrive at a situation where thecarrying capacity is much less than the design capacity because thiscapacity cannot be accessed fully as a result of fragmentation. In suchcircumstances connection control eliminates the corresponding route andthen passes the message back to call routing for forwarding to adestination AGR.

Thus, the destination AGR choice is based on the highest priorityaccessible, reachable, voice route which immediately identifies the AGRfrom which that voice route emanates so from that point on signallingmessages are forwarded to the outgoing (OG) call routing on the otherside of the network. At that stage the outgoing Call Routing isresponsible for determining whether it can make a particular trunkcircuit selection. A voice route consists of many trunk circuits, socall routing has to deal with the possible race conditions of incomingsignalling on that side which may want to select the same voice circuit,because they are both way circuits, from that same voice route. Theremay be various circuit selection policies. Call routing monitors theprogress of those signals and will then perform the final selection ofthe trunk circuit in that given voice route with respect to whetherthere is a race condition, which is termed glare, and also taking intoaccount whether certain circuits are out of service, not provisioned, orthe type of call, e.g. whether it is a test call. Once the circuit hasbeen selected, call routing passes this information to connectioncontrol and begins the third stage of the process to actually connectthe voice path between the end points P and Q, either in aunidirectional or bidirectional manner in the network system, this beingreferred to as a worm which is a specification of the nodes that thevoice path will pass through. The content of the worm specifies theconnection of the circuit at each stage of the connection in turn fromtrunking AVJ to grooming AVJ and back from grooming AVJ to trunking AVJin the other remote AGR. Essentially the mechanism is driven by the wormbeing passed forward in a chain and then passed back as a form ofacknowledgement. Once it has reached the outgoing stage again then thetrunking AVJ responsible passes it to connection control which passesout a message to Call Routing which can then send out the outgoing IAMsignal. This illustration of the worm is in fact a main path which showsthe whole passage of a call set up. The call handling that isencompassed on the AGR has taken into account external and internalcongestion. When the call routing is a proxy call routing, i.e. the AGRwere just providing a connection entity, it is connection control'sresponsibility to handle internal congestion, and the particular form ofcall routing or special services would take into account their own meansof determining external congestion, for the signalling scheme in use,and the handshake between Call Routing and connection control and theselection of the outgoing voice route applies equally to these proxyservers.

FIG. 7 shows how the existing SS7 signalling scheme can be used tosignal messages to and from the AGR network system. In FIG. 7, localexchange LEA is a local exchange but it is also providing a signallingtransfer point for local exchange LEB, so that local exchange LEBsignals its messages through local exchange LEA. The AGR network systemconsists of AGRs 41 to 44. one of which (41) is the incoming AGR.

Outgoing AGRs are illustrated just to illustrate a means for balancingtraffic load to local exchange LEC which happens to be dual homed ontothese two outgoing AGRs. The local exchange LEA can select a destinationpoint code. The LEA when it has performed digit analysis on the callderives a destination point codes (DPC) and from the destination pointcode derives particular signalling route and then a particularsignalling link set and a particular signalling link to reach the AGRsystem layer MTP-3 (Message Transfer Part 3) whereby to determinewhether the messages are indeed for this AGR, which from the DPCdetermines whether the messages are being forwarded to another exchange,either in the AGR system, or external to the AGR system, or whether theyhave in fact been routed incorrectly. From the destination point codeand the originating point code, the MPT-3 can tell whether to handlethis message internally, which is passed to the narrow band ISDN UserPart (NIS UP), and determines whether the message came from localexchange LEB or from local exchange LEA, in which case it may be handledaccordingly, or whether the destination point code differs from the AGRin this case, LEC, in which case there is an STP function wherebymessages are forwarded to MTP3 in one of the outgoing AGRs using SAALwhich is the broadband signalling ATM adaptation layer. If thedestination point code and originating point code imply a signallingroute, the message is passed to N-ISUP, which from the destination pointcode and the originating point code associates that signalling routewith a particular incoming voice route, and the CR entity that takes thesignalling messages and, by digit analysis, determines the possible setof destination point codes, i.e. the point code implies a possible setof outgoing voice routes that connect between the AGR network system anddestination NB exchanges which therefore implies a possible set ofoutgoing AGRs which head-up those voice routes. Once a choice of voiceroute has been made with due consideration to load balancing, internalcongestion, to determine which is the outgoing AGR, a similar schemefrom Call Routing down through N-ISUP down to MTP3 signals the outgoingexchange, for example the local exchange LEC. Thus, the destinationpoint code of the AGR is now replaced with the destination point code ofexchange LEC, the originating point code is now replaced with that ofthe AGR network system rather than exchange LEA or LEB. The destinationpoint code, will imply a particular voice route between that originatingpoint code and the new destination point code, local exchange LEC whichwould imply a particular signalling route and therefore a particularsignalling link set.

Referring now to FIG. 8, this illustrates the way in which call routingprioritises voice routes statically or dynamically according to networkcongestion statistics. Statically means a static prioritisation of routeaccording to the three tier network where there is a first priority, asecond priority and so on, or joint first priority, and dynamicallymeans the use of more advanced algorithms such as dynamic call routingalgorithms.

Call routing passes the set of potential destination AGRs to ConnectionControl. Connection Control is aware of the topology of the AGR networksystem, which AGRs are connected together by virtual trunk groups. Theremay be more than one virtual trunk group (VTG) between any given pair ofAGRs. Therefore there is a choice for load balancing between VTGs, butcombined with the candidate set of AGRs there could be more than one VTGto choose from overall. Connection control maps the set of AGRs onto theset of candidate VTGs which are carried in virtual paths in the ATMsense, referred to as virtual trunk routes, and connection control canestablish from the set of VTGs those voice routes which are notreachable due to internal congestion. By knowing the capacity availableon the VTGs, because these are dynamically sized, connection controlknows from congestion criteria whether it can dilate any particular VTGand what the greatest likelihood of success will be for any givenchoice. This provides a facility to block routes which are unreachablein the network at the outset. By rejecting calls with as small amount ofprocessing as possible then, when the system is heavily overloaded thereis reduced processing and signalling generated for calls which can notcurrentlby be established across the network. By providing thisessentially negative feedback scheme, a call can be rejected at theperiphery of the AGR network before an abortive attempt to route thecall across the network and thus a high degree of carried traffic can bemaintained. Connection control does not necessarily know theprioritisation of voice routes, but it will eliminate those voice routeswhich are not reachable without actually changing the priority andtherefore it can achieve a load balancing capability with voice routeswhich are of equal priority or may have changed their dynamic priorityfrom the call routing perspective. Call routing can thus select onegiven route which has a high degree or certainty of success andtherefore need be only indirectly aware of the internal resourceavailability.

A preferred AGR connection protocol stack is illustrated by way ofexample in FIG. 9.

FIG. 10 shows the final stage of the path set up process at which,having selected the outgoing circuit, with knowledge of the incomingcircuit, the voice path is now to be set up therebetween. Because theAGRs have fixed topology, they have an inter-AGR stage which is shown aszone B in this figure. This is the VTG the virtual trunk group andintra-AGR which is the Virtual Channel Junctor VCJ, which is for mostpurposes identical to the VTG and works on substantially identicalprinciples. In this ATM based domain it is necessary to connect from thetrunking AVJs, which interface with trunk circuits directly tointermediate stages which are called grooming AVJs because they groomtraffic from and to the wide area (the inter-AGR traffic) and finally tothe trunking AVJ which is associated with circuit P. The worm thuscontains five identifying features which are illustrated here as being,the outgoing circuit Q, the next stage which identifies a grooming AVJthat will connect us to our chosen VTG, which therefore implies aparticular VCJ, the VTG chosen by Connection Control and the VCJ whichwill take us to the trunking AVJ which interfaces directly with circuitP. By passing this worm forward from controllers built into the AVJ,they can each set up their connection maps to ensure that there is avoice path continuously from circuit P to circuit Q in either aunidirectional or a bidirectional scheme and that could form a forwardor a reverse voice path. Passing the worm to and fro to one end and thenback is a ready form of acknowledgement of the signalling scheme.

FIG. 11 illustrates the functional topologyof the system and shows amethod for achieving both load balancing through a number differentroutes between AGRs and also for providing physically diverse routes forresilience purposes. Each AGR consists of trunking AVJs, and groomingAVJs which interface to a wide area network which has an ATM switch.This is a non-blocking ATM switch described earlier. Any local trafficin an AGR would simply be passed between its trunking AVJs. Any widearea network would be passed to a grooming AVJ for grooming on to a VTG.To achieve physically diverse paths one must ensure that those pathspass through a minimum of at least two switches in the ATM network.These switches are the two VC cross-connects shown and since a VTG is avirtual channel connection, it can be cross-connected at the VC leveland by ensuring the ability to connect between any pair of AGRs bypassing through at least two VC cross-connects in the network ensures aphysically diverse path because the cross-connects must be distinct, andtherefore use distinct ports, and therefore there is no single point offailure on those routes. In the path between an AGR and a cross-connectthere is a virtual trunk route (VTR) which in ATM terms is a virtualpath. A VTR can pass through any number of intermediate switches in theATM network functioning as virtual path cross-connects which have notbeen shown for clarity, but logically it is a path from AGR to VCcross-connect and whatever bandwidth is associated with the virtualtrunk route as it emanates from the AGR is identical to the capacity ithas arrives at the virtual cross-connect. Thus each VTG and itspotentially physically diverse pair is contained in at most two VTRsbetween any two AGRS. Thus an AGR can determine in its local section ofVTR1 to the left hand side, VTR2 to the right hand side, what capacityhas been negotiated with the ATM network, and what the availablebandwidth is currently for calls in progress. VTGs are carried withinvirtual trunk routes and a virtual trunk route can comprise a number ofVTGs, which are segregated at and multiplexed at the VC cross-connect,hence the utilisation of VTR1 is not necessarily the capacity of VTR2,there being no necessary association. AGR.1 is aware of the capacity andutilisation of VTR1 on the left hand side, and the AGR2 on the righthand side knows the capacity and utilisation VTR2 then the end to endcapacity and utilisation is known through the ATM network, whatever thenumber of intermediate switching stages, and there is no isolatedsection whose capacity and utilisation could not be determined in thismanner, hence the AGRNs can determine the congestion level in thenetwork.

Referring now to FIG. 12, this shows a method of determining the sparecapacity on a virtual trunk route. The virtual trunk route statisticsare processed by a VTR capacity monitor which retains the maximum VTRsize which is the bandwidth/capacity negotiated with the ATM network,and the maximum bandwidth that one could use on the route. Some of thisbandwidth may be reserved for maintenance purposes, but the rest isdevoted to VTG bandwidth. The VTR CU also has information on the VTGsthat the VTR comprises and, by subtracting the current absolute sizes ofthe VTGs from the maximum usable bandwidth of the VTR, results in thespare capacity of the VTR, and whether that route is be congested ornot. The VTR CU is ideally located in the Connection Control layer ofthe AGR control system.

A VTR is a shared resource in the Connection Control, and thereforeinterrogation for its spare capacity can be single threaded on a call bycall basis, providing a ready means to share out the spare capacity. TheVTR CU information could be updated every time a call is modified in aVTG; it comprises as a means to reduce signalling, one could have afixed time interval, for update, between which the VTR CU grants acredit per interval to each VTG it comprises based on that VTG'sabsolute—and change in utilisation in the previous interval, or anyother appropriate scheme is applicable, so in this manner the VTRcapacity monitor need not be precise in operation. When the VTR capacitymonitor has crossed a certain predefined threshold, and there may beseveral such thresholds, it flags processed calls a corresponding degreeof congestion, and thresholds would be determined according to a desirednetwork performance and efficiency. Bandwidth for VTRs may be decidedaccording to desired network performance and those thresholds can beused as an indication of internal congestion in place of or inconjunction with the absolute spare capacity, the indication of havingcrossed the threshold may provide a direct means of comparison andsuitability between candidate VTRs in routing a call, and as a means fordata/signalling compression.

FIG. 13 illustrates the provision of grooming capacity information inthe AVJ device. Whilst there may be spare capacity on a VTR, the VTGs itcomprises could be controlled by distinct physical devices, i.e.grooming AVJs. Consequently whilst there may be spare capacity in theVTR there may no longer be any capacity on a grooming AVJ that controlsa VTG of interest, i.e. a VTG that is candidate for end-to-end routing.A grooming AVJ can signal call-by-call or, on occasion, the currentsizes of the VTGs that it operates to the respective VTR CUs, such thatthey know the size of VTGs, but a grooming AVJ can also send its sparegrooming capacity resource, and associate that spare resource with therelevant and respective VTGs to which it may apply in unequal measuredue to implementation compromises. Thus, the VTR capacity monitor cancheck the quality of a candidate route to VTG resolution, whether it hascrossed a threshold of congestion or whether any of the VTGs havecrossed a threshold of congestion by lack of capacity on their groomingAVJ. VTGs may include padding to ensure constant end-to-end delay in thenetwork. When some or all the channels in a given VTG are not used, byspecifying some minimum number of channels in a VTG, one ensures maximumend-to-end delay, and if those channels are not used then they arepadded, but because they are padded, adding a channel to the VTG wouldnot in fact increase the VTG size, so that capacity extra to the sparegrooming AVJ capacity. Under those situations the AVJ grooming sparecapacity can be ignored. So now we have a complete picture in the VTR'scapacity monitor of any local congestion.

FIG. 14 illustrates the process of advertising local conditions to thefar end through the VTGs, i.e. the AGRs to which a local AGR isconnected. Advertising local congestion in terms of crossed thresholdsor obsolete utilisation can be done on a relatively infrequent basis andit provides the other end with information of the remote VTR sectioncongestion, because being able to dilate and increase a channel in a VTGat the local end is necessary but not a sufficient means to ensure thereis capacity at a remote end. This is a preferred means to ensure to avery high probability that calls will be connected on first attempt bymaking at the outset a proper assessment of whether a particular AGR isreachable by any given path and therefore reject blocked traffic atsource and hence the advantage of no impairment to the amount of carriedtraffic under overload conditions local or remote or universal.

A VTR capacity monitor can advertise its local congestion status interms of thresholds or absolute capacity to all or select remote VTRcapacity monitors to which it is connected, by VTGs through the VTGs itcomprises, by using for example the ATM F5 cell method or any otherequivalent signalling scheme which can be associated with a virtualchannel connection or VTG. A VTRCU can advertise to all those VTRCUs orto a selection, based on any selection criteria, for examplegeographical distance or logical hierarchical placement. The VTGs couldbe given for example a geographic location or, reach or distanceindicator, and thereby the congestion could be sent out as an indicationto only a local area if the distance is below a certain threshold withan associated congestion threshold, thereby only AGRs within a givengeographic locale will get signalled first and as congestion increasesthen it could spread to the wider AGR network system. One can envisageany scheme of selection criteria for any advantageous purpose, which maybe select or universal in application. A VTR capacity monitor obtainsremote VTR congestion indication for any VTG that it comprises and withregard to FIG. 10a, congestion indications of remote grooming capacityby identical means and criteria from a remote VTR CU or remote groomingAVJ, for any VTG it comprises, and it can supply Connection Control anoverall preference by applying any type of weighted cost function to thelocal and remote congestion indicators and Connection Control can decidethereby the best routing policy with relatively up-to-date localinformation and relatively out-of-date advertised remote informationprior to making any call connection, to select a reachable voice routeand then forwarding the signalling remote AGR where its ConnectionControl may re-examine the best VTG route from the remote perspective,where it will have an up-to-date picture of its own congestion and arelatively up-to-date picture of the emanating AGR at the local end anda more up-to-date picture at the point when the final selection of VTGis made to send the worm back across the network to complete the voicepath connection.

As shown in FIG. 15, the scheme described above with reference to FIG.14 can be extended to the provision of voice band processors in anygiven connection. An example of this would be an echo canceller or halfecho canceller, where the exchange in its call routing from thesignalling will decide, because of the length of the route, or otherrelevant criteria on a need to apply echo cancellation for examplebecause the call has or will cross some certain delay budget. Because agrooming AVJ has a natural loop back capability where it simply isgrooming aggregated local traffic for the wide area network, there istraffic between its ATM domain and its TDM domain and then being mappedback into the ATM domain almost immediately, in performing the groomingfunction. This traffic may be diverted most advantageously through avoice band processor, for example a half echo canceller, connecting tothe grooming AVJs, this being the preferred embodiment although allother placements of echo cancellers or voice bands in the end to endconnection may be applied in the ATM or TDM domain. The capacity of thehalf echo cancellers, because they operate by a paired user port andnetwork port, also represent a resource depleted by utilisation. Thegrooming AVJ is aware of and controls the maximum capacity of the halfecho canceller to which it is immediately attached, and duringconfiguration is informed which timeslots the echo canceller isconnected to. Because the grooming AVJ knows the identity of thosetimeslots and whether they have a half echo canceller configured forthem, it can automatically determine what the half echo cancellercapacity is that is connected to it, and it can report this togetherwith its own capacity to the VTR capacity monitor. In the United States,inter-exchange carriers on some of their long-routes always need halfecho cancellers, but they have a statistical distribution of the needfor half echo cancellers. One example would be 30% of all routes havinga need for half echo cancellers. By statistically distributing half echocanceller capability across all the grooming AVJs serving such routes,whatever grooming AVJ is chosen there is a statistical chance of havinghalf echo canceller capability if it is necessary. This has theadvantage over the use of dedicated grooming AVJs for half echocancellers by providing greater utilisation efficiency and avoidance ofresonance depletion through fragmentation. Connection control may selecta VTG without congestion and satisfy the criteria of echo cancellerresource if applicable. The half echo cancellers can be readily enabledand disabled by simple loop back control within the grooming AVJ whichis another means of inclusion or exclusion of the half echo canceller inthe path and this is the preferred means of accommodating signallingschemes which introduce echo cancellers at the outset of a call and thenexclude them subsequently through subsequent signalling to ensureoptimal placement closest to called party. The grooming AVJs canoptimise this function by finding other timeslot means to loop backwhich do not utilise the half echo canceller resources. The VTR capacitymonitor is thus a single reference for all the resources necessary toconnect a particular voice path through the network system.

Referring now to FIGS. 17 to 20, these show in sequence the means foractually setting up the voice path between trunk circuits P and Q.

This incorporates a 5 part protocol for signalling over to a cascade ofan arbitrary number of multiple stages, Each stage comprises an ingressand a egress process defined with respect to the ATM domain, egressbeing traffic that emanates from the ATM domain into the TDM domain inall cases. The egress process keeps a record of free channel timeslotsand therefore dictates where the offset in any given trunk group, VTG orVCJ of a new channel is going to be by determining where a free timeslotis in relation to the other active channels already in the trunk groupand therefore the offset into that group when the new channel is added.A similar process applies to the criteria for removal of an existingchannel from the trunk group. Using the offset the egress process cansignal to the ingress process which is responsible for assembly of thattrunk group into ATM cells, and the ingress process is responsible foradding the channel through the dynamic structure data transfer functionof our earlier patent, and the egress process can detect the occurrenceof changes in trunk group size and therefore knows that the channel hasbeen connected and can receive subsequently further changes on thatparticular VTG cor VCJ. This operation process, firstly is singlethreaded by VTG or VCJ, and the figure illustrates the two parts of theprocess, firstly signalling an offset from egress to ingress, andsecondly making the change to trunk group size, and forwarding the nextstage's offset as a fully pipelined process in a unidirectional manner,in which, connecting a voice path in the upstream direction from P to Qis achieved by signalling in the reverse direction from Q to P which isa preferred embodiment, because it minimises the signalling transactionsinvolved, but does not imply the exclusion of signalling in the samedirection as the voice circuit and moreover the unidirectional mechanismmay be used to effect a bidirectional mechanism by signalling first fromQ to P and then in an identical manner for the reverse downstream voicepath signalling from P to Q. Once an egress process has signalled aningress process, the ingress process can transfer signalling to theegress process in the same AVJ for example, the grooming AVJ at stage 2which in turn determines the offset it needs for the VTG between stage 3and 2 and signals to that ingress process in stage 3 and so on throughthe system. The offset may be embodied in a cell referred to as a wormwhich passes to and fro to set up a bidirectional voice path. Provisionis made for AVJ stages 2, 3 and 4 to reject the connection because thecongestion indication given by the VTR capacity monitor could be out ofdate with respect to the true capacity status in any AVJ because ofdelay race for-resource conditions in the AGR network system. Therefore,a backtracking capability is built into the AVJ connection entities andprovision is made for the means to function correctly in the absence ofan advertising mechanism in part or in total. Provision is also made forcorrect operation in the absence of a VTR capacity monitor by abacktrack capability or similar recovery means. The AVJs at stages 1 and2, may optionally have capability to send a digital signature therebyguarding against any false simulation of that identity intentionally orotherwise. This provides a means of authentication such that eachtrunking AVJ at the end of a communication path knows that it hasconnected the voice path through to AGR network correctly providinganalogous integrity checks in existing narrow-band exchanges which have,for example, a additional integrity pattern or other signalling schemesto ensure proper cross-connection of narrow band channels through thefabric. We envisage sending a digital signature for unambiguouslyidentifying the identities of the trunking AVJs at each end, and thecircuits P and Q prior to or subsequent to completion of the voice path,in the bandwidth of that voice path on erection, or prior to dissolutionof that voice path in order to provide a means to ensure continualintegrity checking on operations pertaining to network connectivity. Thegrooming AVJs are free to choose any free time slot in their connectionmap. This need not be prescribed by the worm, but the worm can beannotated with that information so that connection control software canreadily use the returned worm acknowledgement to check that the AVJdevices are performing the intended function. This is not a strictrequirement, there still being a means for disconnection of any trunkcircuit Q from any trunk circuit P by a voice path trace operation.

FIG. 18 shows an example of the main contents of the worm. At the top ofthe diagram are illustrated functional stages in which TID meansTrunking Identity, GID means Grooming Identity and the suffix denotesthe local and remote ends. The remote end AGR is the incoming end of thecall, and the local end is the outgoing end of the call since the wormbegins at the outgoing end. The incoming half echo canceller and theoutgoing half echo canceller, are positioned relative to the respectiveends of the call. The incoming side is the left hand side, the remoteside, because call routing forwarded messages to the right hand side andthe preferred embodiment is to send information in the backwardsdirection initially to minimise transactions. The items in heavy typeare those items which are necessary to connect a unique path through thesystem It should be noted here that the remote trunking time slot P andthe remote trunking AVJ identity can be forwarded from the remote end aspart of a call routing signalling message and no other topologyinformation regarding the remote AGR is required. For example, underfault or maintenance a remote AGR could independently change itsconfiguration, and the information forwarded need only be valid on acall-by-call basis. There is a window of opportunity of call-by-callresolution for making configuration changes without affecting any othercalls in progress in the system. The trunking AVJ ID at the local end,the trunking time slot and the grooming identity which heads theselected VTG are all locally stored topological information. Theinclusion or exclusion of echo cancellers is a call-by-call decisionprocess. The central part of FIG. 18 shows that in the AGR networksystem which connects to peripheral exchanges, any signalling forinclusion or exclusion of incoming or outgoing echo cancellers isassociated with the respective side of the call, the exchange on thelocal side only affects incoming echo cancellers. The exchange on theremote side can be arranged to signal only for outgoing half echocancellers and this avoids the need to store any information orconfiguration supervision of echo cancellers in the remote AGR. andlocal information need be retained during the lifetime and dissolutionof a call. The lower portion of FIG. 18 shows three main types of wormoperation namely, the connect, the disconnect and the modify commands.The connect command is for setting up the initial voice path and theitems which are greyed out are those which can be annotated by thegrooming AVJs and stored by connection control if necessary, but whichare not absolutely necessary for the system operation. In total thesecomprise an indication of inclusion or exclusion half echo cancellersand a specification of the exact route that the voice path should take.This facilitates the setting up of point to multi-point connections inthe AGR network system. The disconnect command can optionally includethe local information which is circled in the figure. In apoint-to-multi-point connection, then this information would be neededto specify unambiguously the grooming AVJ identity and there may be morethan one voice path through that same grooming AVJ, distinguished by thegrooming time slots, which must be passed back from the original connectmessage. In a point-to-point connection using for example SS7signalling, these circled pieces of information are not necessarybecause any time slot P to any other time slot Q has just one uniqueconnection. A disconnect ion can be facilitated by a bare minimum ofinformation of just one end of the connection, for example timeslot Q,and both are not necessary to be known as described herein. The notepadarea is used for annotation as the worm is passed forward, for exampleto hold the offset into the trunk group between each successive AVJstage. It is distinct from the annotated information uniquelyidentifying the voice path such as the choice of a grooming time slot.The modify command is associated with further subsequent inclusion orexclusion of echo cancellers in the voice path, which is just onespecific example of path modification, all types of modification areenvisaged, in this particular case it may be desirable to move agrooming time slot away from a half echo canceller device to a groomingtime slot which is a simple loop-back function or vice versa, to free upresources by inverting the original echo canceller indication with anoptional facility to specify the grooming time slot and the worm couldbe subsequently annotated with the new time slot, specifying at minimum,the thinking time slot, the grooming AVJ identity, the indication ofhalf echo canceller and for point-to-multipoint connections above thegrooming time slots and the VTG.

FIG. 19 shows the way in which a voice path is traced through the AGRnetwork system. The trunking time slot is common to both the ingress andthe egress process for a both-way circuit, and is readily available inthe TDM domain. Hence the trunking time slot for both ingress and egressprocesses with respect to the ATM is known such that, at step 1, thetrunking timeslot location in the egress connection map contains aparticular VCJ identity and the connection entity for that VCJ theoffset of that trunking time slot in the VCJ trunk group, Trunking TimeSlot (T.TS) in the upstream direction. That offset can be passed to thenext AVJ stage, the grooming AVJ. Since VCJs are configured asreciprocating pairs, at step 2, knowing the upstream VCJ also meansknowing the downstream VCJ. The ingress process can then, from the VCJconnection map, determine the offset of the trunking time slot T.T.S(step 3) in that particular VCJ, the offset downstream, offset D. (step4) The offset of the upstream and the downstream direction in thetrunking AVJ have now been determined and can be passed by signalling asdescribed above to the grooming AVJ in the next stage of the connection.At the grooming AVJ, from the offset in the upstream direction in thegiven VCJ, the ingress process at step 2 can determine the groomingtimeslot in the upstream direction (step 3) which is the timeslot usedin the upstream direction on the VCJ side and on the VTG side. From thegrooming timeslot in the upstream direction the egress process candetermine the VTG from the contents of the location in the map and in anidentical manner to the trunking AVJ stage, can determine the offset ofthat grooming timeslot in the VTG trunk group, the offset in theupstream direction (step 3). From the offset in the downstream theegress process can determine the particular grooming timeslot in thedownstream trunk group that occurs at that offset (step 5) andconsequently from complementary pair of the upstream VTG, it determinesthe downstream VTG at step 4. The ingress process can thus determine theoffset in the downstream direction of that grooming timeslot in thatparticular downstream VTG also at step 5, and thus the offset in thedownstream direction and the offset in the upstream direction can besignalled to the next grooming or trunking stage, and the process cancontinue in a cascade in a similar manner so as to determine thecomplete voice path through to the trunk circuit P at the remote end.This provides resilience of the system, for as long as the hardware isreliable. Even where the software could be largely out-of-date, it couldalways recover the hardware state and determine what voice paths areestablished in the network, and resume full operation subsequently.

FIG. 20 shows the overview of the whole wormhole protocol. Step 1 showsthe ITU SS7 message, i.e. the IAM message identifying trunk circuit P.This also contains dialled digits which call routing processes,determines the voice route by consideration for external congestion anda prioritised set of external voice routes (step 2) which it passes toconnection control at step 3. Connection control checks the internalcongestion from the VTR capacity monitors and, if necessary, (step 4),connection control may also initiate any SS7 loop back continuity checkson trunk circuit P or any other type of continuity check appropriate tothe signalling system employed. Connection control eliminates thoseroutes which are unreachable through internal congestion and putsforward its preferred route for load balancing where there is an equalexternal voice route. Call routing determines the final choice of voiceroute and forwards the message. At step 5 the outgoing call routingselects the outgoing trunk circuit and resolves any glare (contentionfor trunk circuits that may be coming in from its own incoming processon this AGR). Once the circuit is selected, connection control checksinternal congestion again through the VTR capacity monitors and makes afinal selection of the VTG that connects the associated pair of AGRs andcreates a worm which comprises sufficient topological information toconnect trunk circuit Q to trunk circuit P via the chosen VTG andforwards this at step 7 to the connection entity inside the trunking AVJhosting Q, which may also initiate a continuity check or any otherscheme provided by the signalling system on trunk circuit Q step 8. Theworm is passed forward through the grooming AVJ stages which mayoptionally include or exclude half echo cancellers in this pathinitially, and they may be excluded at any later stage by a modifiedworm. The grooming AVJ forwards the worm through the VTG to the remoteAGR on the incoming side (step 9) which connects the path through totrunk circuit P and also then passes the worm back in the reversedirection (step 10) to set up in the reverse direction. As a form ofacknowledgement, the annotated information as to what the path was inthe worm information is stored in connection control at both ends sothat the path can be torn down from either end using any of the existingschemes in the signalling system. Once the worm has passed back to theoutgoing side it is passed back to connection control and call routingwhich will then signal an outgoing IAM message (step 11) so as to enablethrough-connections which are shown as switches in FIG. 20. It thusfacilitates the connection of a voice path through this single largedistributed exchange system. Step 12 is an optional stage to check theintegrity of the voice path to enable the through connection and theoutgoing IAM message.

The arrangement and method described provide for the distribution of theexchange in terms of its fabric and its control and its enablingtechnologies with reference to the dynamic structure data transfer.There is uncommitted bandwidth within the fabric and consequentlyrouting decisions can be fully independent of the operation of thisdistributed fabric. The distributed fabric can, because of thisunallocated bandwidth use separate connections to the control layer forestablishing the connections through the fabric and this in no waycompromises external decisions that are made except when an overloadsituation is encountered. An advertising process provides knowledge ofthe distributed fabric. This includes local knowledge about a remotesite such that routing decisions can be made and modified wherenecessary so as to reject traffic at source. The dynamic trunkingenables the separation of call routing and connection control. Thearrangement also provides a means of ensuring stability under overloadsituations and minimising the cost of handling traffic which would berejected by destinations.

The separation of call routing and connection control together withadvertising the system status ensures a wide range of scalability sothat the application of dynamic trunking technology provides scalabilityin a traffic sense. Further, the separation of call routing andconnection control provides a distributed computing environment which isscaleable and managed by this advertisement resource. Because thedistributed exchange manages its own internal traffic, effectively itprovides means for balancing that traffic to the fabric and makes itsown internal routing decisions. Once that network grows through itsscalability, a local exchange can dual home on to it and allow it tomake its own routing decisions. The local exchange does not have to makeany individual routing decisions to a variety of traffic exchanges. Theseparation further provides a facility to support a wide range ofservices using other signalling schemes. The fabric provides aconnection engine that can accommodate a wide variety of signallingprotocols appropriate for the type of service to be provided and thatcan set up connections.

The full knowledge of the network connectivity and the release ofresources can be enabled by any node within the network, as it can tracethrough all connections from any starting point. This can be used tosupport failure recovery.

It will be appreciated that although the arrangement and method havebeen described above with particular reference to current standardprotocols, such as SS7 signalling, it is in no way limited to the use ofthese particular protocols.

What is claimed is:
 1. A telecommunications system comprising: anasynchronous transfer mode (ATM) network; and a plurality of adaptivegrooming routers (AGRs) coupled to the network, each AGR comprising anATM switch including at least one adaptive virtual junctor (AVJ) foradapting narrowband traffic received at said AGR to/from the ATMadaptation layer, wherein the AGRs are interconnected across the ATMnetwork by virtual trunks and are arranged to function as a distributednarrowband exchange to set up narrowband connections across the ATMnetwork.
 2. A telecommunications system as claimed in claim 1, wherein aplurality of local exchanges are coupled to the network and wherein thelocal exchanges are dual homed to the network.
 3. A telecommunicationssystem as claimed in claim 2, wherein the network incorporates means forload balancing of traffic originating from the local exchanges.
 4. Atelecommunications system as claimed in claim 1, wherein the virtualtransit exchange comprising the AGRs incorporates independent connectioncontrol and call routing functions.
 5. A telecommunications system asclaimed in claim 4, wherein the call routing function is adapted toperform a selection of potential voice routes and to prioritise thatselection.
 6. A telecommunications system as claimed in claim 5, whereinsaid prioritisation is performed from an assessment of congestion ofsaid potential voice routes.
 7. A telecommunications system as claimedin claim 4, wherein the service and connection aspects of the connectioncontrol function are separated whereby to facilitate support of advancedservices and a plurality of connection models.
 8. A telecommunicationssystem as claimed in claim 5, wherein said call routing functionincorporates means for determining availability of a destination and forrejecting at source traffic to that destination in the event that thedestination is unavailable.
 9. A telecommunications system as claimed inclaim 1 and incorporating means for providing a narrow band multicastfunction.
 10. A telecommunications system as claimed in claim 1 andincorporating means for voice path tracing whereby to effect failurerecovery.
 11. A telecommunications system as claimed in claim 1including means for determining the current traffic status of the systemwhereby to effect routing of narrow band traffic across the system. 12.A method of routing telecommunications traffic in a system comprising:an asynchronous transfer mode (ATM) network; and a plurality of adaptivegrooming routers (AGRs) coupled to the network, each AGR comprising anATM switch including at least one adaptive virtual junctor (AVJ) foradapting narrowband traffic received at said AGR to/from the ATMadaptation layer, wherein the AGRs are interconnected across the ATMnetwork by virtual trunks and are arranged to function as a distributednarrowband exchange to set up narrowband connections across the ATMnetwork.
 13. A method as claimed in claim 12, wherein a set of potentialvoice routes are determined for a connection and wherein aprioritisation of said routes is performed from an assessment ofcongestion of said potential voice routes.
 14. A method as claimed inclaim 13, wherein the availability of a destination is determined andtraffic to that destination is rejected at source in the event that thedestination is unavailable.
 15. A method as claimed in claim 12, whereinthe current traffic status of the system is determined prior toeffecting routing of narrowband traffic across the system.
 16. Atelecommunications system comprising: an asynchronous transfer mode(ATM) network; and a plurality of adaptive grooming routers (AGRs)coupled to the network, each AGR comprising an ATM switch includingmeans for adapting narrowband traffic received at said AGR to/from theATM adaptation layer, wherein the AGRs are interconnected across the ATMnetwork by virtual trunks and are arranged to function as a distributednarrowband exchange to set up narrowband connections across the ATMnetwork.